1. Field of the Invention
The present invention is directed to a novel composition which forms color and is rendered electrically conductive on heating, and to a heat sensitive image recording sheet using the same.
2. Description of the Prior Art
A variety of heat sensitive recording elements are known and described in detail, for example, in Jaromir Kosar, Light Sensitive Systems, Chapter 9, John Wiley & Sons (1965), Masanobu Wada, Insha Kogaku II, Chapter 5, Kyoritsu Shuppan Co., Ltd., Tokyo (1969), etc. These conventional heat sensitive recording elements are broadly classified as follows. In one group the element contains a recording layer which undergoes a physical change due to the action of heat to thereby form images. The element uses, for example, a substance which melts on heating and becomes transparent, or a colored substance which sublimes on heating and is thereby transfered into an image receiving layer, or the like. In another group the recording element is one in which a chemical reaction is induced on a recording layer by the action of heat, and as a result, colored visible images are formed.
A heat sensitive recording element which belongs to this group includes, firstly, an element in which two or more reactive components are present in but separated in the recording layer so that they do not react with each other at room temperature and, by heating, they are brought into contact and reacted to thereby form a colored material. As reactive components, combinations of metal compounds such as fatty acid metal salts, etc., and reducing agents or chelate-forming agents, which produce colored images comprising reduced metal or metal chelate compounds due to heating, and further, combinations of dye precursors such as leuco dyes and reducing agents which produce dye images due to heating are used. Secondly, an element which forms colored substances directly or indirectly on thermal decomposition thereof are present. For example, a combination of a compound which releases an alkaline substance due to the thermal decomposition thereof, a pH indicator or a diazonium salt and a coupler; a combination of phenols and nitroso compounds, etc.; are known. In each case, the thermal decomposition products react with a second substance to produce a colored image.
Various disadvantages are encountered with these chemical methods since all utilize relatively unstable low molecular weight organic compounds or metal compounds as the image forming components. For this reason, the recording elements are generally unstable at room temperature and in addition, many of these substances are toxic or expensive. Further, the images formed are unstable to light or heat where the images are dye images. Furthermore, dye images and metal chelate images do not provide a sufficiently high image density. Also, in a recording element comprising two or more kinds of color forming components, it is necessary for these components to be separated so that they do not react with each other at room temperature and as a result most of the recording layers are generally opaque per se.
It is known that the splitting off of hydrogen chloride from halogen-containing polymers (for example, polymers of vinyl chloride or vinylidene chloride) is induced due to the action of light or heat and as a result polyenes are formed to thereby form a color. Many investigations of these reactions have been made mainly from the viewpoint of stabilizing halogen-containing polymers and the results of recent investigations are reported, for example, in G. Ayrey, B. C. Head and R. C. Poller, "Macromolecular Reviews," J. Polymer Sci., vol. 8, page 1 (1974).
Further, light image recording elements which utilize that property that a color is formed due to the lack of stability of halogen-containing polymers, particularly due to conversion into polyenes have been proposed in Japanese Patent Publications Nos. 10647/75 and 35451/74, U.S. Pat. Nos. 3,615,456, 2,772,159, 3,046,136, 2,712,996 and 2,754,210, and S. B. Elliot, Photographic Eng., vol. 4, page 133 (1953). All of these elements involve a system in which a substance which, by the action of light, accelerates the hydrogen halide splitting off reaction of the halogen-containing polymers is added and polyene images are formed only by exposure or by heating subsequent to exposure.
Furthermore, heat sensitive image recording elements utilizing the above-described property of halogen-containing polymers have been proposed in U.S. Pat. Nos. 2,855,266 and 3,155,513. In these systems, metal compounds are added to the halogen-containing polymers as a hydrogen halide splitting off accelerator. These heat sensitive elements have the disadvantage that the recording layer thereof is opaque since particles of metal compounds (for example, metal oxides) are added.
On the other hand, it is reported, for example, in H. Wechster, J. Polymer Science, vol. 11, page 233 (1953) and L. A. Matheson and R. F. Boyer, Ind. Eng. Chem., vol. 44, page 867 (1952) that halogen-containing polymers react in solution with bases such as pyridine or morpholine and polyenes are formed. A method for preparing ion exchange resins utilizing this reaction has been proposed in British Pat. No. 697,503. However, these methods are all directed to solution reactions, which are not appropriate for heat sensitive recording elements.